Multi-layer steel cords are known in the art, particularly in the art of tire making. Multi-layer steel cords may reinforce both the breaker or belt plies of a truck tire and the carcass plies of a truck tire. The main function of steel cords in the breaker or belt plies of a tire is to give stiffness to the tire. As the breaker plies of a tire can be replaced by retreading, the mechanical properties required from steel cords in the breaker ply are not extremely high. In great contrast herewith, the mechanical properties required from steel cords for the reinforcement of carcass plies are much higher. One of the reasons is that carcass plies are not replaced during the life time of a tire. As a consequence, a higher strength, a higher degree of flexibility, more structural uniformity, a higher adhesion and adhesion retention level, and a higher durability are expected from steel cords for carcass plies. The higher durability can be expressed as a higher resistance against fatigue and a higher resistance against fretting.
Following prior art steel cord construction has been widely used as reinforcement for the carcass plies of truck tire:3+9+15×0.175+1 (wrapping filament)SSZS
Tests show that this cord has a bad fretting performance. This is mainly due to the presence of the wrapping filament and due to the point contacts between filaments of different layers, more particularly between filaments of a S-twisted and filaments of a Z-twisted layer.
Omission of the wrapping filament leads to following prior art construction3+9+15×0.175 SSZand to some improvement regarding fretting.
Yet another alternative prior art steel cord which has been tried is following construction:3+8+13×0.175
The difference with the previous construction is that there is one filament omitted in the first layer and two filaments omitted in the second layer. This omittance leads to a decreased degree of compactness with unsaturated layers which allow rubber to penetrate. The penetrated rubber reduces the fretting damage between the filaments.
The best fretting behaviour, however, has been noticed with following prior art construction:0.20+18×0.175 CC (compact cord).
This construction comprises one thicker core filament and 18 thinner filaments which are twisted with the core filament in the same twisting direction and with the same twisting step. As a result of this twisting geometry, line contacts instead of point contacts are present between the filaments. An additional advantage is reached by the thicker core filament which leads to slightly unsaturated layers which allow rubber penetration. The line contacts together with the rubber penetration are the causes for the good fretting behaviour. However, the core filament remains the most vulnerable element.